Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers can be fabricated from piezoelectric materials or can be a new form of ultrasonic transducer known as a micro-machined ultrasonic transducer (MUT). Piezoelectric transducer elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer assembly. MUT's are typically fabricated using semiconductor manufacturing techniques with a number of elements typically formed on a common substrate to form a transducer assembly. Regardless of the type of transducer element, the transducer assembly is then further assembled into a housing possibly including control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the PZT transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
Typically, the transducer elements within the ultrasonic probe are excited by a voltage signal, which causes each transducer element to emit an interrogation pulse, or beam. It is possible to control the application of the voltage signal to each transducer element within the array through the use of delay circuitry associated with each transducer element and thereby direct the resulting beam. After interrogating the target, the pulse is reflected back to the ultrasonic probe where the transducer elements, operating in a receive mode, receive the ultrasonic energy, convert the ultrasonic energy to electrical signals, and pass the electrical signals to receive circuitry. The receive circuitry analyzes the received energy and constructs an image for presentation to a viewer.
Oftentimes, it is desirable to steer and focus the ultrasonic energy transmitted to a target and received from the target. To steer and focus the beam on transmit, the transmit delays are adjusted to equalize the time for a pulse to propagate to the desired focal point in the image plane substantially perpendicular to the transducer elements at a defined distance from the center of the surface of the transducer array and at a defined angle to a line normal to the surface of the transducer array.
To steer and focus the beam on receive, the receive delays are adjusted to equalize the time for a pulse to propagate from the desired focal point. For both transmit and receive, a contiguous subset, or aperture, of the available transducer elements are enabled. A larger aperture results in a narrower beam and reduced depth of field. Often the transmit aperture is smaller than the receive aperture since dynamic receive focusing (fine adjustment of the receive delays as the wave front moves deeper) makes depth of field a greater concern on transmit than receive.
To display an image, control circuitry causes the transducer elements to pulse repeatedly using different transmit and receive delay settings to achieve different angles. As this is done the properties of the ultrasound beam change subtly. Looking directly forward with respect to the probe, both the transmit and the receive beams typically have higher amplitude, are narrower, and have less depth of field than they do at off-angles. The term "off-angles" refers to angles formed by the transmit or receive of a beam that is steered to angles other than substantially perpendicular to the plane of the transducer element.
Unfortunately, the effect of larger off-angles typically provides a beam having reduced power and reduced resolution, resulting in a weaker image at off-angles. One manner of improving image uniformity at off-angles is to increase transmitter voltage and/or receive amplifier gain at off-angles. Note, however that beam width and depth of field may remain uncorrected. Also, when gain is increased, the system's sensitivity to thermal noise is also increased resulting in more noise in the image at off-angles.
The above discussion assumes linear effects. Increasingly, non-linear effects such as the bursting of contrast bubbles or the generation of harmonics by bubbles or tissues are being exploited by ultrasound systems. Images based on non-linear effects are even more vulnerable to non-uniformities due to transmit amplitude than images based on linear effects. For example, received second harmonic signals can be expected to decrease by two dB for each reduction of one dB in transmit amplitude. This particular problem can be addressed by increasing transmit voltage at the off-angles. However, this requires a difficult and expensive power supply to accomplish and leaves other transmit beam issues than amplitude as well as all receive beam issues not addressed.
Therefore, it would be desirable for a steered beam emanating from and received by an ultrasonic transducer array to exhibit uniform characteristics at all angles.